Selective oxidation methods and transistor fabrication methods

ABSTRACT

The invention includes selective oxidation methods and transistor fabrication methods. In one implementation, a selective oxidation method includes positioning a substrate within a chamber. The substrate has first and second different oxidizable materials. The substrate is therein exposed to a gas mixture comprising an oxidizer and a reducer under conditions effective to selectively grow an oxide layer on the first material relative to the second material. The oxidizer oxidizes the first and second materials under the conditions. The reducer reduces oxidized second material under the conditions back to the second material. After selectively growing the oxide layer on the first material relative to the second material, partial pressure of the oxidizer and the reducer is reduced within the chamber by flowing an inert gas to the chamber while chamber pressure and chamber temperature are at or above those of the conditions during the exposing. Other aspects and implementations are contemplated.

RELATED PATENT DATA

This patent resulted from a continuation application of U.S. patentapplication Ser. No. 10/688,784, filed Oct. 17, 2003, now U.S. Pat. No.7,235,497 entitled “Selective Oxidation Methods and TransistorFabrication Methods”, naming Don C. Powell as inventor, the disclosureof which is incorporated by reference.

TECHNICAL FIELD

This invention relates to selective oxidation methods and to transistorfabrication methods.

BACKGROUND OF THE INVENTION

One type of circuitry device is a field effect transistor. Typically,such includes opposing semiconductive material source/drain regions ofone conductivity type having a semiconductive channel region of oppositeconductivity type therebetween. A gate construction is receivedproximate the channel region, typically between the source/drainregions. The gate construction typically includes a conductive regionhaving a thin dielectric layer positioned between the conductive regionand the channel region. Current can be caused to flow between thesource/drain regions through the channel region by applying a suitablevoltage to the conductive portion of the gate.

Typical transistor fabrication methods include a step referred to assource/drain re-oxidation. Such may be conducted by any of a number ofreasons depending upon the materials, sequence and manner by whichtransistor components have been fabricated prior to the re-oxidationstep. For example, one method of providing a gate dielectric layer is tothermally grow an oxide over a bulk or semiconductor-on-insulatorsubstrate. In certain instances, source/drain regions are provided byconducting ion implantation through this oxide layer after the gateconstruction has been patterned to at least partially form thesource/drain regions. The heavy source/drain implant is likely to damageand contaminate the oxide remaining over the source/drain regions. Evenif all the oxide were removed over the source/drain regions prior to theimplant, damage to the crystal lattice and the source/drain outersurface typically occurs from the source/drain implant(s). Accordinglyand regardless, a re-oxidation step is conducted to grow a fresh,uncontaminated oxide on the source/drain regions towards repairingcertain damage caused by the implant. This typically occurs after anyremaining damaged oxide has been stripped from over the source/drainregions.

Typically, this re-oxidation also grows a very thin thermal oxide ontops and sidewalls of the conductive components of the gateconstruction. Further, it tends to slightly thicken the gate oxide underthe gate corners, and thereby round the lower outer edges of the typicalpolysilicon material of the gate. The ion implantation and any oxidestripping can weaken or mechanically compromise the gate oxide at thesidewall edges of the gate, and tend to increase the field effecttransistor gate-to-drain overlap capacitance. The thickening androunding of the gate oxide at the corners can reduce gate-to-drainoverlap capacitance, and relieve the electric-field intensity at thecorner of the gate structure, thus enhancing the gate oxide integrity atits edge. Further, the thermal oxide can serve as a dopant diffusionmask preventing dopant diffusion from subsequently deposited insulativeinterlevel dielectric layers.

However in many instances, it is desirable that none or a minimum ofcertain conductive materials of the transistor gates be oxidized. Forexample, one presently employed gate construction uses polysilicon,tungsten nitride and elemental tungsten as conductive materials. Whenusing steam as a source/drain re-oxidant, the conditions would also tendto significantly oxidize the elemental tungsten. A prior art techniqueto minimize the effective formation of tungsten oxide on the tungsten isto provide H₂ in combination with steam in the oxidizing atmosphere. TheH₂ tends to reduce the tungsten oxide back to tungsten, thus reducing orminimizing the amount of tungsten oxide which forms on the sidewalls ofthe tungsten material.

Some of the tungsten oxide which forms is in the vapor phase, with theoxidation/reduction reaction essentially being one of equilibrium withH₂ and H₂O. Unfortunately, tungsten oxide tends to deposit on internalreactor surfaces largely at the conclusion of the source/drainre-oxidation. Such requires periodic cleaning of the internal reactorcomponents, thus reducing production time of the reactors. One prior arttechnique for increasing the time between cleanings is to reduce flow ofsteam to the reactor prior to reducing flow of the H₂.

The invention was motivated in addressing the above described issues,but however is in no way so limited. The invention is only limited bythe accompanying claims as literally worded (without interpretative orother limiting reference to the above background art description,remaining portions of the specification or the drawings) and inaccordance with the doctrine of equivalents.

SUMMARY

The invention includes selective oxidation methods and transistorfabrication methods. In one implementation, a selective oxidation methodincludes positioning a substrate within a chamber. The substrate hasfirst and second different oxidizable materials. The substrate isexposed within the chamber to a gas mixture comprising an oxidizer and areducer under conditions effective to selectively grow an oxide layer onthe first material relative to the second material. The oxidizeroxidizes the first and second materials under the conditions. Thereducer reduces oxidized second material under the conditions back tothe second material. After selectively growing the oxide layer on thefirst material relative to the second material, partial pressure of theoxidizer and the reducer is reduced within the chamber by flowing aninert gas to the chamber while chamber pressure and chamber temperatureare at or above those of the conditions during the exposing.

In one implementation, a transistor fabrication method includes forminga transistor gate comprising semiconductive material and conductivemetal. Source/drain regions are formed proximate the transistor gate.The transistor gate and source/drain regions are exposed to a gasmixture comprising H₂O and H₂ within a chamber under conditionseffective to oxidize outer surfaces of the source/drain regions. Afteroxidizing the outer surfaces of the source/drain regions, partialpressure of the H₂O and the H₂ within the chamber is reduced by flowingan inert gas to the chamber while chamber pressure and chambertemperature are at or above those of the conditions during the exposing.

Other aspects and implementations are contemplated.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below withreference to the following accompanying drawings.

FIG. 1 is a diagrammatic sectional view of an exemplary substratefragment at a processing step in accordance with an aspect of theinvention.

FIG. 2 is a view of the FIG. 1 substrate at a processing step subsequentto that shown by FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This disclosure of the invention is submitted in furtherance of theconstitutional purposes of the U.S. Patent Laws “to promote the progressof science and useful arts” (Article 1, Section 8).

Preferred embodiments of selective oxidation methods are described withreference to FIGS. 1 and 2. the invention was reduced-to-practice in thefabrication of field effect transistors, as is described with referenceto FIGS. 1 and 2. However, the invention has applicability in selectiveoxidation methods involving any substrate, and in the fabrication of anycircuitry or non-circuitry component or device.

Referring initially to FIG. 1, a semiconductor substrate is shown. Inthe context of this document, the term “semiconductor substrate” or“semiconductive substrate” is defined to mean any constructioncomprising semiconductive material, including, but not limited to, bulksemiconductive materials such as a semiconductive wafer (either alone orin assemblies comprising other materials thereon), and semiconductivematerial layers (either alone or in assemblies comprising othermaterials). The term “substrate” refers to any supporting structure,including, but not limited to, the semiconductive substrates describedabove. Also in the context of this document, the term “layer”encompasses both the singular and the plural unless otherwise indicated.

The semiconductor substrate comprises an exemplary bulk substratematerial 12, for example monocrystalline silicon. Of course, othermaterials and substrates are contemplated, includingsemiconductor-on-insulator and other substrates whether existing oryet-to-be developed.

A transistor gate construction 10 is formed over substrate 12. By way ofexample only, such includes a conductive transistor gate 14 sandwichedbetween a pair of dielectric layers 16 and 18. Dielectric layer 16serves as a gate dielectric, with a preferred exemplary material beingthermally grown silicon dioxide having a thickness of from 25 Angstromsto 70 Angstroms. Typically, insulative layer 18 serves as an insulativecap, with exemplary preferred materials being silicon nitride and/orundoped silicon dioxide provided to an exemplary thickness of from 700Angstroms to 1,100 Angstroms. Transistor gate 14 comprises at least asemiconductive material and a conductive metal. In the context of thisdocument, a “metal” includes any of an elemental metal, an alloy of atleast two elemental metals, and metal compounds whether stoichiometricor not stoichiometric. Example preferred metals include W, Pt, Co, Mo,Pd, Cu, Al, Ta, Ti, WN, and conductive metal oxides, by way of exampleonly. Further by way of example only, exemplary semiconductive materialsinclude conductively doped silicon, for example polysilicon.

The exemplary embodiment transistor gate 14 is illustrated as comprisingthree layers 20, 22 and 24. An exemplary material 20 is conductivelydoped polysilicon deposited to an exemplary thickness of from 250Angstroms to 750 Angstroms. An exemplary material for layer 22 istungsten nitride provided in a 1:1 atomic ratio of tungsten to nitrogen,and to an exemplary thickness range of from 80 Angstroms to 100Angstroms. An exemplary preferred material for layer 24 is elementaltungsten deposited to an exemplary thickness range of from 200 Angstromsto 400 Angstroms.

The illustrated layers 16, 20, 22, 24 and 18 would typically besuccessively formed over a substrate and then collectively patterned toform the illustrated gate construction 10, for example byphotolithography and tech. FIG. 1 depicts source/drain regions 26 and 28being formed proximate transistor gate 14. In the context of theillustrated preferred embodiment and invention, the formation of thesource/drain regions at this point in the process might be only partialor might be complete. For example, source/drain regions of field effecttransistors typically include multiple different concentration, andsometimes even conductivity type, implants. Examples include LDD,primary highest dose implants, halo regions, etc. Typical exemplarymethods of forming such regions include ion implantation, gas dopantdiffusion, and out-diffusion from adjacent solid materials. In thecontext of this document, any degree of formation of the source/drainregions is contemplated, whether partial or complete, and whether by anyexisting or yet-to-be developed processes. Further and by way of exampleonly, some, all or none of gate dielectric layer 16 might be removedlaterally outside of the pattern depicted by gate construction 10 priorto one or more of the processing which results in the partial orcomplete formation of the source/drain regions.

The substrate would be positioned within a suitable chamber. In abroader context of the invention related to selective oxidation methods,the substrate, as positioned within the chamber, would comprise firstand second different oxidizable materials that will be oxidized to somedegree, as described below. By way of example only with respect to theFIG. 1 embodiment, one or both of materials 20 and 12/26/28 could beconsidered as a first oxidizable material, with layer 24 constituting anexemplary second different oxidizable material.

Within the chamber, the substrate is exposed to a gas mixture comprisingan oxidizer and a reducer under conditions effective to selectively growan oxide layer on the first material relative to the second material. Inthe context of this document, selective growth on one material relativeto another is defined as a differential growth rate on the one of atleast two times the growth rate on the another. Under the conditionswithin the chamber, the oxidizer oxidizes the first and secondoxidizable materials. The reducer reduces at least some of the oxidizedsecond material under the conditions back to the second material. Theselected conditions for the exposing will, in large part, depend uponthe first and second different oxidizable materials on the substrate, aswell as upon the oxidizer and reducer which are utilized. Of course, theuse of more than one oxidizer or more than one reducer is alsocontemplated.

By way of example only, consider substrate material 12/26/28 asconstituting a first oxidizable material and an elemental tungsten layer24 as constituting a second oxidizable material. Such are shown in theillustrated FIG. 1 example as having at least some portion thereofoutwardly exposed. However, the invention also contemplates none of suchlayers being outwardly exposed as long as any covering material isdiffusive of the oxidizer and reducer under the conditions to at least adegree to enable the stated oxidizing and reducing effects. Regardless,an exemplary oxidizer in such a system is H₂O and an exemplary reduceris H₂. Under suitable conditions, the H₂O will tend to oxidize both thesilicon of materials 12/26/28 and polysilicon 20, as well as theelemental tungsten of layer 24. Some of the oxidized tungsten will tendto form a layer (not shown) on the sidewalls of layer 24, as well asform WO_(x) in vaporized form within the chamber. However under suitableconditions, the H₂ will have a greater tendency to reduce the WOx backto elemental tungsten such that very little solid WOx forms on thesidewalls of layer 24. As described below, reduction-to-practiceexamples resulted in a 50:1 and greater selective deposition of an oxideon the silicon surfaces as compared to those of the tungsten surfaces.

By way of example only, exemplary alternate different second oxidizablematerials include tantalum and hafnium containing materials, whiledifferent alternate first oxidizable materials include gallium arsenideand silicon-germanium alloys. Further by way of example only, anexemplary alternate oxidizer utilizable with such materials would beCO₂, with an exemplary alternate reducer for such materials being CO.

Exemplary preferred conditions include rapid thermal processing.Further, the conditions might comprise a pressure below, at or greaterthan room ambient pressure. When greater than ambient room pressure, inone preferred embodiment, the pressure is no greater than 1.25 timesroom ambient pressure in Torr and, in a more preferred embodiment nogreater than 1.21 times room ambient pressure in Torr. Of course,significantly higher pressures are also contemplated. However, apreferred reason for operating at such pressures only slightly elevatedfrom ambient room pressure is to preclude room ambient oxygen fromentering the chamber in the event of a leak, which might introduceprocessing variability and/or safety issues. Further, operating at suchpreferred slightly elevated pressures might in certain instances enablesuch processing advantages while using processing equipment primarilydesigned to operate at room ambient pressure conditions.

FIG. 2 depicts a preferred result in the formation of oxide layers 30and 32 over source/drain regions 26 and 28. Such rounds the outerlateral edges of gate oxide layer 16, and oxidizes the polysiliconsidewalls, forming oxide regions 40. Preferably, very little if anyoxide forms on the sidewalls of the exemplary exposed tungsten layer 24.

In a first implementation and regardless of pressure, after selectivelygrowing the oxide layer on the first material relative to the secondmaterial, partial pressure of the oxidizer and the reducer are reducedwithin the chamber by flowing an inert gas to the chamber while chamberpressure and chamber temperature are at or above those of the conditionsduring the exposing. The gas mixture might be void of any inert gasduring the exposing immediately prior to the partial pressure reducing.Alternately, the gas mixture might comprise inert gas during theconditions immediately prior to the partial pressure reducing. In suchlatter instance, the partial pressure reducing would comprise increasingthe flow of inert gas (either with the same, fewer, or additional inertgases) to the chamber from what it was immediately prior to the partialpressure reducing. As identified in the background section, the priorart is recognized to have reduced partial pressure of the oxidizer onlyby reducing the flow of the oxidizer to the chamber, but not by the actof flowing an inert gas to the chamber.

In one aspect, after the partial pressure reducing by the act of flowingan inert gas (and meaning regardless of whether inert gas is flowingimmediate prior to the partial pressure reducing), the flow of thereducer to the chamber is reduced prior to reducing the flow of theoxidizer to the chamber. In such instance, the flow of the reducer tothe chamber might be stopped completely prior to reducing the flow ofthe oxidizer to the chamber, or not stopped completely prior to reducingthe flow of the oxidizer to the chamber.

Alternately by way of example only, after the partial pressure reducingby the act of flowing an inert gas, aspects of the invention contemplatereducing the flow of the oxidizer to the chamber prior to reducing theflow of the reducer to the chamber. Further, the flow of the oxidizer tothe chamber might be stopped completely prior to reducing the flow ofthe reducer to the chamber, or not stopped completely prior to reducingthe flow of the reducer to the chamber. Alternately by way of exampleonly, after the partial pressure reducing by the act of flowing an inertgas, the flow of the oxidizer and the reducer to the chamber might bereduced simultaneously.

In one implementation, where the conditions comprise pressure greaterthan room ambient pressure, the method further comprises reducingpressure to below room ambient pressure after the partial pressurereducing. In such implementation, the flow of the reducer to the chambermight be reduced prior to the pressure reducing, including reducing thereducer flow to zero prior to the pressure reducing. Further, the flowof the oxidizer might be reduced prior to the pressure reducing,including reduction to zero flow. Further, both reducer and oxidizermight be reduced prior to reducing the pressure to below room ambientpressure after the partial pressure reducing, or neither might bereduced.

In one exemplary reduction-to-practice example, processing in accordancewith the invention was carried out in an Applied Materials Centura, 3.5Liters, single wafer, rapid thermal processor. An exposed refractorymetal was on the substrate, as well as exposed silicon. An argon purgeof the chamber was conducted initially at a flow of from 5 slm to 10 slmfor from 5 to 30 seconds at a substrate temperature of below 300° C. anda chamber pressure of from 1 Torr to 2 Torr. H₂ flow to the chamber wasadded after 20 seconds under the purge conditions. A preferredvolumetric flow would be from 30% to 50% by volume of hydrogen comparedto the total quantity of hydrogen and argon flowed to the chamber.Thirty percent (30%) was utilized in a preferred reduction to practiceexample. The substrate was then heated to 900° C., with an exemplarypreferred temperature range being from 800° C. to 1050° C. Thetemperature increase ramp rate was approximately 75° C./second. Pressurewas also increased to 820 Torr, while room ambient pressure was 680Torr. Such processing preferably continues for from 1 second to 10seconds. Argon flow to the chamber was then turned off, and H₂O flow tothe chamber was introduced. The preferred volumetric flow of the H₂O isfrom 5% to 50% by volume of the combined H₂ and H₂O flows, with 35% H₂Obeing used in a preferred reduction to practice example. Temperature wasmaintained at 900° C., with chamber pressure at 820 Torr. An exemplarypreferred processing time is anywhere from 5 seconds to 5 minutes toresult in selective oxide growth, for example as shown in FIG. 2.

After the selective oxide growth has been accomplished to a degreedesired, an inert gas, such as argon, was flowed to the chamber whilechamber pressure and chamber temperature are at or above those of theconditions during the exposing, for example in the preferred embodimentexample while the chamber was at 900° C. and the pressure was at 820Torr. In the specific reduction-to-practice example, argon was flowed tothe chamber until reaching an argon flow rate of 10 slm while flowingthe H₂O and H₂ at their previous volumetric ratio relative to oneanother, for example at a 65:35 ratio of H₂ to H₂O. Such argon flowthereby reduced the partial pressure of the oxidizer and the reducerwithin the chamber by the act of flowing the inert gas thereto. Ofcourse if inert gas were flowing immediately prior to the partialpressure reducing, such act could be effected by increasing total inertgas flow to the chamber. An exemplary desireable or preferred argon flowrate is 70% by volume, as compared to the flow rate of all gases to thechamber. In one preferred implementation, the argon was brought up to astable flow rate in a short period of time, for example from 2 to 3seconds. In such preferred embodiment, the H₂ flow is then preferablyreduced to zero over 1 to 10 seconds, with 5 seconds being a specificreduction-to-practice example. Thereafter, in one preferredimplementation, chamber pressure is brought down to below room ambientpressure, for example, and by way of example only, to a pressure of from600 Torr to 1 Torr while the argon and H₂O continues to flow to thechamber. A reason for doing so would be to improve removal of residualgases, including for example WO_(x).

Thereafter, flow of the H₂O to the chamber was turned off. Accordinglyin this reduction-to-practice example, flow of the oxidizer and thereducer to the chamber is ultimately reduced to zero. Thereafter ifdesired, the H₂ as a reducer can be fed to the chamber. A possiblereason for doing so would be towards reducing any remaining solidtungsten oxide back to tungsten.

In one preferred implementation, the chamber could then be cooled downto below the exposing conditions which produced the selectiveoxidization.

The above provides but one, preferred, reduction to practice example.

In a second implementation, a selective oxidization method comprisesexposing the substrate within the chamber to a gas mixture comprisingthe oxidizer and the reducer under conditions effective to selectivelygrow an oxide layer on the first material relative to the secondmaterial, where the conditions comprise pressure greater than roomambient pressure. After selectively growing the oxide layer on the firstmaterial relative to the second material, partial pressure of theoxidizer and the reducer is reduced within the chamber by flowing in aninert gas to the chamber while chamber pressure is greater than roomambient pressure, and independent of maintenance or changing of thetemperature to above or below that of the conditions during theexposing. After the partial pressure reducing, chamber pressure isreduced to below room ambient pressure within the chamber while flowingthe reducer and the oxidizer to the chamber. All other preferred aspectsof the processing could be conducted as described above.

The above-described processing can be utilized in transistor fabricationmethods, for example as described and shown. Further, the inventioncontemplates transistor fabrication methods employing H₂O and H₂regardless of selective oxidization issues, and regardless of whetherconductive metal of the transistor gate is oxidizable under theconditions which oxidize outer surfaces of source/drain regions. Forexample, in one implementation, the invention contemplates exposing thetransistor gate and source/drain regions to a gas mixture comprising H₂Oand H₂ within the chamber under conditions effective to oxidize theouter surfaces of the source/drain regions. After oxidizing the outersurfaces of the source/drain regions, partial pressure of the H₂O andthe H₂ within the chamber is reduced by the act of flowing an inert gasto the chamber while chamber pressure and chamber temperature are at orabove those of the conditions during the exposing. Of course, all otherpreferred aspects of the processing could be conducted as describedabove.

Further in one implementation, the invention contemplates exposing thetransistor gate and the source/drain regions to a gas mixture comprisingH₂O and H₂ within a chamber under conditions effective to oxidize theouter surfaces of the source/drain regions, where the conditionscomprise pressure greater than room ambient pressure. After oxidizingthe outer surfaces of the source/drain regions, partial pressure of theH₂O and the H₂ within the chamber are reduced by the act of flowing aninert gas to the chamber while chamber pressure is greater than roomambient pressure. After such partial pressure reducing, chamber pressureis reduced to below room ambient pressure while flowing H₂O and H₂ tothe chamber, for example at, above or below the flowing amounts duringthe exposing conditions. Of course, all other preferred aspects of theprocessing could be conducted as described above.

In compliance with the statute, the invention has been described inlanguage more or less specific as to structural and methodical features.It is to be understood, however, that the invention is not limited tothe specific features shown and described, since the means hereindisclosed comprise preferred forms of putting the invention into effect.The invention is, therefore, claimed in any of its forms ormodifications within the proper scope of the appended claimsappropriately interpreted in accordance with the doctrine ofequivalents.

1. A selective oxidation method comprising: providing a substrate withina chamber, the substrate comprising first and second differentoxidizable materials, the first and second materials having coveringmaterial thereover; exposing the substrate within the chamber to a gasmixture comprising an oxidizer and a reducer under conditions effectiveto selectively grow an oxide layer on the first material relative to thesecond material, the oxidizer and reducer diffusing through at leastsome of said covering material with the oxidizer oxidizing the first andsecond materials under the conditions, the reducer reducing oxidizedsecond material under the conditions back to the second material; andafter selectively growing the oxide layer on the first material relativeto the second material and while exposing the substrate to the oxidizerand the reducer within the chamber, reducing partial pressure of theoxidizer and the reducer within the chamber by flowing an inert gas tothe chamber while chamber pressure and chamber temperature are at orabove those of the conditions during the exposing.
 2. The method ofclaim 1 wherein the first material comprises silicon and the secondmaterial comprises a metal in at least one of elemental or alloy forms.3. The method of claim 1 wherein the gas mixture is void of any inertgas immediately prior to said partial pressure-reducing.
 4. The methodof claim 1 wherein the gas mixture comprises inert gas immediately priorto said partial pressure-reducing, said partial pressure-reducingcomprising increasing inert gas flow to the chamber from what it wasimmediately prior to said partial pressure-reducing.
 5. The method ofclaim 1 wherein the conditions comprise a pressure below room ambientpressure.
 6. The method of claim 1 wherein the conditions comprisepressure greater than room ambient pressure.
 7. The method of claim 1wherein after the partial pressure-reducing by flowing an inert gas,reducing flow of the reducer to the chamber prior to reducing flow ofthe oxidizer to the chamber.
 8. The method of claim 1 wherein after thepartial pressure-reducing by flowing an inert gas, reducing flow of theoxidizer to the chamber prior to reducing flow of the reducer to thechamber.
 9. The method of claim 1 wherein after the partialpressure-reducing by flowing an inert gas, reducing flow of the oxidizerand the reducer to the chamber simultaneously.
 10. The method of claim 1wherein the conditions comprise pressure greater than room ambientpressure, the method further comprises reducing chamber pressure tobelow room ambient pressure after said partial pressure-reducing.
 11. Aselective oxidation method comprising: providing a substrate within achamber, the substrate comprising first and second different oxidizablematerials, the first and second materials having covering materialthereover; exposing the substrate within the chamber to a gas mixturecomprising an oxidizer and a reducer under conditions effective toselectively grow an oxide layer on the first material relative to thesecond material, the oxidizer and reducer diffusing through at leastsome of said covering material with the oxidizer oxidizing the first andsecond materials under the conditions, the reducer reducing oxidizedsecond material under the conditions back to the second material, theconditions comprising pressure greater than room ambient pressure; afterselectively growing the oxide layer on the first material relative tothe second material and while exposing the substrate to the oxidizer andthe reducer within the chamber, reducing partial pressure of theoxidizer and the reducer within the chamber by flowing an inert gas tothe chamber while chamber pressure is greater than room ambient pressureand while chamber temperature is at or above that of the conditionsduring the exposing; and reducing pressure to below room ambientpressure within the chamber after said partial pressure-reducing whileflowing the reducer and the oxidizer to the chamber.
 12. The method ofclaim 11 wherein the gas mixture is void of any inert gas immediatelyprior to said partial pressure-reducing.
 13. The method of claim 11wherein the gas mixture comprises inert gas immediately prior to saidpartial pressure-reducing, said partial pressure-reducing comprisingincreasing inert gas flow to the chamber from what it was immediatelyprior to said partial pressure-reducing.
 14. The method of claim 11wherein the conditions pressure is no greater than 1.25 times roomambient pressure in Torr.
 15. A transistor fabrication method,comprising: forming a transistor gate comprising semiconductive materialand conductive metal; forming source/drain regions proximate thetransistor gate; exposing the transistor gate and source/drain regionsto a gas mixture comprising an oxidizer and a reducer within a chamberunder conditions effective to oxidize outer surfaces of the source/drainregions; and after oxidizing the outer surfaces of the source/drainregions and while exposing the substrate to the oxidizer and the reducerwithin the chamber, reducing partial pressure of the oxidizer and thereducer within the chamber by flowing an inert gas to the chamber whilechamber pressure and chamber temperature are at or above those of theconditions during the exposing.
 16. A transistor fabrication method,comprising: forming a transistor gate comprising semiconductive materialand conductive metal; forming source/drain regions proximate thetransistor gate; exposing the transistor gate and source/drain regionsto a gas mixture comprising an oxidizer and a reducer within a chamberunder conditions effective to oxidize outer surfaces of the source/drainregions, the conditions comprising pressure greater than room ambientpressure; and after oxidizing the outer surfaces of the source/drainregions and while exposing the substrate to the oxidizer and the reducerwithin the chamber, reducing partial pressure of the oxidizer and thereducer within the chamber by flowing an inert gas to the chamber whilechamber pressure is greater than room ambient pressure and while chambertemperature is at or above that of the conditions during the exposing;and reducing pressure to below room ambient pressure within the chamberafter said partial pressure-reducing while flowing the oxidizer and thereducer to the chamber.